Electrically conductive adhesives are widely used in the electronics industry. Important applications include die-attach and assembly of printed wiring boards (PWBs). An example of a device, 2, with a quad arrangement of leads, 3, surface mounted on a PWB, 4, to a corresponding plurality of conductor pads, 5, which, in turn, are connected to conductors, 6, on the PWB forming a part of a larger assembly, 1, is shown schematically in FIG. 1. As device dimensions continue to shrink, the numbers of leads on devices to be connected to the PWBs continue to increase and the distance between centers of adjacent leads or pads (known as a "pitch") continues to decrease, it becomes more difficult to accurately place isotropically conductive adhesives without shorting adjacent leads. An alternative approach is to use anisotropically conductive adhesives which conduct only in the direction perpendicular to the board (z-direction).
Anisotropically conductive adhesives, hereinafter called AdCons (for Adhesive Connectors), is a class of electrically conductive adhesive materials formulated on the "bridging" concept. The materials are prepared by dispersing electrically conductive particles in an insulating polymer matrix forming an adhesive composite. Typically, the adhesive composite is applied to the surface of a PWB by stencil printing, screen-printing or laminating a film of the conductive adhesive. A schematic representation of a portion of PWB, 4, with conductor pads 5, a region of AdCon, 7, on the PWB and a device 2 with leads 3 to be connected to the conductor pads, is shown in FIG. 2. After the device is placed onto the PWB and a placement force displaces the AdCon from between the conductor pads on the PWB substrate and the leads on the device, a layer of AdCon with a thickness of a single particle remains between each lead and conductor pad, as is shown schematically in FIG. 3. Individual particles span the gap between the device and PWB and form an electrical interconnection. Similarly, semiconductor chips with conductive pads may be surface mounted on the PWB.
In the past, the formation of AdCon interconnections was typically conducted via serial processes where each package was aligned, placed under pressure, and cured individually in a curing oven. Examples of such processing may be seen from U.S. Pat. No. 4,667,401 issued on May 26, 1987 and U.S. Pat. No. 4,868,637 issued on Sep. 19, 1989, both to James R. Clements et al. or from an article by Brian Sun "`The Paste Connector`--Vertically Conductive Adhesive", Connection Technology, August 1988, pp. 31-32. However, this type of processing leads to an inadequate control of applied pressure which could vary from insufficient with formation of non-conducting joints to excessive with resultant damage of leads and interconnections. These processes were difficult to implement both for high input/output (I/O) surface-mount packages and for very small chips, not only because of their low throughput, but also because the assembly processes require coplanarity of the PWB, the package, and an assembly implement. Non-planarity in the system can lead to problems with yield and reliability performance. Furthermore, very small semiconductor chips need an application of high forces per small areas, without the danger of damaging the chips. Therefore, there is a need for an efficient manner for overcoming the above-noted problems, including a need for a process of applying a pressure uniformly across a device or devices of differing dimensions and heights, as well as for an apparatus for implementing the process.